Moulding shrinkage

All rubber products exhibit shrinkage after cure, mainly due to the thermal expansion which occurs at vulcanization temperature. Moulded rubber goods are never as big as the moulds in which they are cured. The difference between the dimensions at room temperature of the finished goods and of the mould expressed as a percentage is called the shrinkage from mould dimensions. 

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Since the material is amorphous the moulding shrinkage is low and normally less than 0.008 cm/cm. 

Glass-reinforced grades of SAN exhibit a modulus several times that of the unfilled polymer and, as with other glass-filled polymers, a reduced coefficient of thermal expansion and lower moulding shrinkage. The materials are thus of interest on account of their high stiffness and dimensional stability. 

Being amorphous, the materials have a low moulding shrinkage (0.044-0.008 cm/cm). 

The absence of crystallisation gives polymers with low mould shrinkage. Molecular orientation. 

The manufacturers stress ease of processing as a particular feature of the material. Recommended melt temperatures are in the range 320-340°C and mould temperatures are 135-165°C. Mould shrinkage of glass-filled grades is usually of the order of 0.2-0.4% in the flow direction and up to twice this value in the transverse direction. The materials are notable for their ability to withstand vapour phase and infrared soldering processes. 

There is little crystallisation on cooling and after-crystallisation has not been observed. Mould shrinkage is consequently of the order of 0.006-0.008 cm/cm and is the same both along and across the flow. 

In the case of glass-filled polymers, moulding shrinkage is somewhat lower (0.003-0.005 cm/cm). 

One particular feature of PPO is its exceptional dimensional stability amongst the so-called engineering plastics. It has a low coefficient of thermal expansion, low moulding shrinkage and low water absorption, thus enabling moulding to close tolerances. 

Good dimensional stability (and low moulding shrinkage)—thus allowing the production of mouldings with close dimensional tolerances. 

A low moulding shrinkage (0.005-0.007 cm/cm) in unfilled grades down to about 0.002 cm/cm in 30% glass-fibre-filled grades. 

Dried granules may be injection moulded, blow moulded and extruded without undue difficulty with melt temperatures of 220-260°C. The moulding shrinkage (0.003-0.004 cm/cm) is very low and a somewhat greater than normal draft may be required in deep draw moulds. 

Another amorphous terephthalate has also been introduced commercially. This is a copolymer based on 1,4-cyclohexylene glycol with a combination of terephthalic acid and isophthalic acid. The copolymer is claimed to have excellent optical properties, have low moulding shrinkage and be relatively easy to vaccum form. As with other terephthalates it is considered in more detail in Chapter 25. 

In some moulding compositions other special purpose ingredients may be incorporated. For example, naphthalene, furfural and dibutyl phthalate are occasionally used as plasticisers or more strictly as flow promoters. They are particularly useful where powders with a low moulding shrinkage are required. In such formulations a highly condensed resin is used so that there will be less reaction, and hence less shrinkage, during cure. The plasticiser is incorporated to... 

The growth of polyesters is hindered by their sensitivity to hot water above 60°C, fire behaviour except for FR grades, mould shrinkage for crystalline products, opacity (except amorphous PET), need of UV protection, limited chemical resistance, higher cost than commodities... 

Mould shrinkage and coefficient of thermal expansion also vary by several percent and can lead to warpage. 

In practice, an average value of the expansion coefficient over a given temperature range is often taken. This is the case in crude measures of mould shrinkage (see Section 3.5 of Chapter 7) where expansion, or rather contraction, of the rubber is the main contribution to the property measured. 

Moulding shrinkage The difference in dimensions, expressed in inches per inch, between a moulding and the mould cavity in which it was moulded, both the mould and the moulding being at normal room temperature when measured. 

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